Thanks everyone. It does, then, appear to be quite common.
I still wonder though, why I was allocated a seat at LHR which was then flagged 'red' to the couple who checked in at AMS, who eventually took my seat (!).
Apart from that faff, the service was fine, at least they treated pax as adults and didn't 'insist' on closing the window blinds, as happened to me with other airlines. But that's another story..